In the printing field, image forming systems exhibiting photographic characteristics including ultrahigh contrast (especially gamma values of 10 or higher) are available in order to improve the reproduction of continuous tone images or halftone images or the reproduction of line images. From the standpoints of environmental protection and space saving, it is strongly desired to reduce the amount of waste solution. Recent research efforts achieved noticeable reduction of waste solution. In systems using processing solutions of chemicals, it is impossible to eliminate the waste solution. Accordingly, the printing field waits for the practical implement of the thermographic system which is simple and ecologically safe since it eliminates the use of processing solutions of chemicals.
There have been proposed image forming processes using photothermographic materials entailing a developing step by heat treatment. Such materials are disclosed, for example, in JP-B 4924/1968 and 6582/1969, JP-A 6074/1971, 97523/1973, and 2781/1995, and U.S. Pat. No. 5,468,603. These photothermographic materials, however, are not suited for the manufacture of printing plates because of low gamma or soft gradation.
In the printing field, photographic characteristics ensuring ultrahigh contrast are desired as described above. The desired ultrahigh contrast is accomplished using hydrazine derivatives as disclosed in U.S. Pat. No. 5,496,695. Where hydrazine derivatives are used, however, stable images are not obtainable since the sensitivity largely changes due to changes of temperature and time of heat development. An improvement in this regard is desired.
Fog by heat development is also a crucial problem. A number of proposals have been made for reducing the fog of thermographic silver halide photosensitive materials. For example, U.S. Pat. No. 3,589,903 discloses mercury salts. There are also known carboxylic acids such as benzoic acid and phthalic acid from U.S. Pat. No. 4,152,160; benzoylbenzene acid compounds from U.S. Pat. No. 4,784,939; indan and tetralin carboxylic acids from U.S. Pat. No. 4,569,906; dicarboxylic acids from U.S. Pat. No. 4,820,617; heteroaromatic carboxylic acids from U.S. Pat. No. 4,626,500; halogenated compounds from U.S. Pat. Nos. 4,546,075, 4,756,999, 4,452,885, 3,874,946 and 3,955,982; halogen molecules or heterocycles associated with halogen atoms from U.S. Pat. No. 5,028,523; palladium compounds from U.S. Pat. No. 4,103,312 and GB 1,502,670; iron group metals from U.S. Pat. No. 4,128,428; substituted triazoles from U.S. Pat. Nos. 4,123,374, 4,129,557 and 4,125,430; sulfur compounds from U.S. Pat. Nos. 4,213,784, 4,245,033 and JP-A 26019/1976; thiouracils from U.S. Pat. No. 4,002,479; sulfinic acids from JP-A 123331/1975; metal salts of thiosulfonic acid from U.S. Pat. Nos. 4,125,403, 4,152,160 and 4,307,187; combinations of metal salts of thiosulfonic acid with sulfinic acid from JP-A 20923/1978 and 19825/1978; and thiosulfonates from JP-B 50810/1987, JP-A 209797/1995 and 43760/1997. None of these patents describe whether or not these compounds are effective for suppressing changes under different heat development conditions of the sensitivity of ultrahigh contrast photosensitive material systems using ultrahigh contrast enhancers.
For example, U.S. Pat. Nos. 4,125,403, 4,152,160, 4,307,187, and JP-A 20923/1978 and 19825/1978 disclose that thiosulfonic acid salts are used in photothermographic materials for preventing fog during heat development or fog generated during shelf storage. None of these patents describe whether or not these antifoggants are effective for suppressing changes under different heat development conditions of the sensitivity of photothermographic material systems using ultrahigh contrast enhancers. EP 762196A discloses a photothermographic material using a hydrazine derivative as a ultrahigh contrast enhancer and describes in Examples to use a thiosulfonic acid salt as an antifoggant. Since the thiosulfonic acid salt is added in a small amount relative to silver, it is not effective for improving sensitivity changes due to changes of heat development temperature and time.
JP-A 82944/1994 discloses a photographic silver halide material to be developed with a liquid developer, comprising a hydrazine derivative and a metal salt or ester of thiosulfonic acid. It is described that the addition of the thiosulfonic acid salt or ester is effective for improving the stability of photographic characteristics of the photosensitive material during shelf storage. It is described nowhere whether or not the thiosulfonic acid salt or ester is effective in photothermographic systems.